1. Field of the Invention
The present invention generally relates to a dropout correcting apparatus for correcting a dropout of a reproduced video signal and, more particularly, is directed to a dropout correcting apparatus suitable for correcting a dropout of a video signal which is reproduced from a video disc.
2. Description of the Prior Art
FIG. 1 is a circuit block diagram showing an example of a prior art reproducing circuit for a video disc. Referring to FIG. 1, reference numeral 1 designates a video disc which is rotated, reference numeral 2 designates an optical pickup device which picks up a signal recorded on the video disc 1, reference numeral 3 designates a demodulating circuit which demodulates the reproduced video signal, reference numeral 4 designates a dropout correcting circuit which corrects a dropout of the reproduced video signal and reference numeral 5 designates a time base fluctuation correcting circuit which removes a jitter component of the video signal.
Further, reference numeral 6 designates an audio signal demodulating circuit connected to the optical pickup device 2 and 7 an RF modulator connected to the time base fluctuation correcting circuit 5.
Since the video signal recorded on the video disc 1 is generally reproduced in the form of a standard composite video signal (of, for example, NTSC system), it is corrected for dropout by the dropout correcting circuit 4 and then fed to the time base fluctuation correcting ciruit 5, in which its jitter component is removed and delivered as a normal and correct video signal. Further, an audio signal is demodulated by the audio demodulating circuit 6.
In such prior art video signal reproducing circuit, as shown in FIG. 1, the dropout of the video signal is corrected by the dropout correcting circuit 4 under the condition that the video signal contains the jitter component or the dropout of the video signal is corrected by a dropout correcting circuit 8 which is connected at the output stage of the time base fluctuation correcting circuit 5 as shown by a broken line in FIG. 1.
FIG. 2 is a block diagram showing a main part of the dropout correcting circuit 4 which carries out the dropout correction prior to the time base fluctuation correcting circuit (hereinafter simply referred to as TBC) 5. In FIG. 2, reference numeral 4a designates a delay element which corrects the time base fluctuation of a video signal one horizontal scanning period (1H) before. This delay element 4a is formed of a CCD (charge coupled device) the delay time of which can be varied by a clock signal CK supplied thereto. Reference numeral 4b designates a PLL (phase locked loop) circuit for producing the clock signal CK which drives the delay element 4a. Reference numeral 4c designates a dropout detecting circuit which produces a dropout flag when it detects a dropout. Reference numeral 4d designates a switching circuit which is changed in position by the dropout flag supplied thereto from the dropout detecting circuit 4c thereby to produce the video signal 1H before only during the dropout period.
In this dropout correcting circuit 4, if the normal video signal having no dropout is supplied thereto, such video signal is supplied as it is through the switching circuit 4d to the TBC 5, while such video signal passed through the switching circuit 4d is supplied to the delay element 4a, in which it is delayed by the time 1H.
Accordingly, if a video signal having a dropout is reproduced next time, the movable contact arm of the switching circuit 4d is connected to the opposite side by the dropout flag derived from the dropout detecting circuit 4c so that the video signal having the dropout is replaced by the video signal 1H before only during the dropout period and the dropout portion thereof is corrected, which is fed to the TBC 5.
FIG. 3 is a circuit block diagram showing a main part of a prior art dropout correcting circuit as disclosed in a published document of Japanese patent application unexamined NO. 120383/1983, in which the dropout correction is carried out after the TBC. Referring to FIG. 3, reference numeral 11 designates a TBC which carries out the time base correction of the video signal, 12 a dropout detecting circuit, 13 a dropout correcting circuit formed of a switching circuit 14 and a 1H delay circuit 15, and 16 a variable type delay circuit which carries out the time base fluctuation correction for the dropout flag in synchronism with the TBC 11 to thereby synchronize the dropout flag with the video signal and which is formed of a CCD or a second time base fluctuation correcting circuit (TBC), etc.
In this dropout correcting circuit, the normal video signal having no dropout is corrected for time base fluctuation by the TBC 11 and then fed via the switching circuit 14 as it is.
If on the other hand the video signal contains the dropout, the dropout is detected by the dropout detecting circuit 12 and the dropout flag therefrom is supplied to the switching circuit 14 through the delay circuit 16 whose delay time is controlled in synchronism with the TBC 11. As a result, the input to the switching circuit 14 is selectively changed by this dropout flag, so that during the dropout period, the normal video signal 1H before derived from the 1H delay circuit 15 is delivered as a corrected signal, thus the dropout being interpolated.
In the prior art dropout correcting apparatus as mentioned hereinabove, when the dropout correction is carried out before the time base fluctuation is corrected, as shown in FIG. 2, it is necessary to provide the variable delay element 4a for delaying the normal video signal which is used in replace of the signal to be corrected in the switching circuit 4d and the PLL circuit 4b which drives this delay circuit 4a in order that the normal video signal and the signal to be corrected are synchronized with each other.
On the other hand, when the dropout correction is carried out after the time base fluctuation is corrected, as shown in FIG. 3, the dropout detecting signal must be corrected for time base fluctuation so as to be synchronized with the signal to be corrected and then fed to the switching circuit 14. Therefore, there is a defect that an extra system of the delay circuit 16 will have to be provided.
For this reason, the prior art dropout correcting apparatus needs many circuit elements and is high in a manufacturing cost and is poor in circuit maintenance.